(1) FIELD OF THE INVENTION
The present invention relates in general to oxygen detectors and in particular to a differential thermocouple device adapted to measure free oxygen in a combustible atmosphere.
(2) DESCRIPTION OF THE PRIOR ART
Oxygen detectors are known which utilize an oxygen ion condutive solid electrolyte, such as zirconium oxide, to sense oxygen content in process gases and combustion flue gases. A system utilizing such a sensor is illustrated in U.S. Pat. No. 3,960,500 issued to Ross et al. Such sensors require elevation of the sensor temperature to its active zone in order to provide a signal indicative of the oxygen content in the gas sample. The required sensor operating temperature may be in excess of 1500F. Obviously this type of sensor is unsuitable for detecting free oxygen in a combustible atmosphere since its operating temperature would be in excess of the auto-ignition temperature of the combustible atmosphere. In addition, the high operating temperature of such sensors could cause the free oxygen to react with the combustible atmosphere prior to actually being detected thereby resulting in a lower, false indication of free oxygen in the combustible atmosphere. This false indication may result in potentially dangerous levels of free oxygen in the combustible atmosphere going undetected.
Prior differential thermocouples detector, such as illustrated in U.S. Pat. No. 4,063,898 issued to Fisher, have been used to monitor combustible gases in an airstream. Such detectors include a differential thermocouple pair with one junction coated with a catalyst and the other junction with a non-catalyst. Combustible gases are heated above the existing ambient temperature of the atmosphere to react with the catalyst to liberate heat to the catalyst-coated thermocouple junction thereby raising the temperature of the catalyst-coated junction above that of the non-catalyst coated junction in proportion the concentration of combustible gases. The output signal from such a device is thus indicative of the concentration of combustible gases in the airstream. However, the applicant is unaware of any prior art combustibles detectors using such differential thermocouples being adapted to measure free oxygen in a combustible atmosphere at the existing ambient temperature of the atmosphere.
It has thus become desirable to develop a detector that will monitor free oxygen in a combustible atmosphere while at the same time eliminating the prior art problem of high sensor operating temperatures that may be excess of the auto-ignition temperature of the combustible atmosphere or result in a false indication of the free oxygen level in the combustible atmosphere.